Complete Ch. 2, Begin Ch. 14, Look at Ch. 11 Lecture #7 OUTLINE Thevenin/Norton Eq. Cont’d Max power transfer theorem The operational amplifier (“op amp”) Feedback Comparator circuits Ideal op amp Unity-gain voltage follower circuit Reading Complete Ch. 2, Begin Ch. 14, Look at Ch. 11

The Operational Amplifier The operational amplifier (“op amp”) is a basic building block used in analog circuits. Its behavior is modeled using a dependent source. When combined with resistors, capacitors, and inductors, it can perform various useful functions: amplification/scaling of an input signal sign changing (inversion) of an input signal addition of multiple input signals subtraction of one input signal from another integration (over time) of an input signal differentiation (with respect to time) of an input signal analog filtering nonlinear functions like exponential, log, sqrt, etc

Op Amp Circuit Symbol and Terminals V + positive power supply non-inverting input + – output inverting input V – negative power supply The output voltage can range from V – to V + The positive and negative power supply voltages do not have to be equal in magnitude.

Op Amp Terminal Voltages and Currents All voltages are referenced to a common node. Current reference directions are into the op amp. V + ic+ ip + – io + vp – in + vo – + vn – ic- V – – Vcc + + Vcc – common node (external to the op amp)

Op Amp Voltage Transfer Characteristic The op amp is a differentiating amplifier: Vcc slope = A >>1 vid = vp–vn -Vcc Regions of operation: “negative saturation” “linear” “positive saturation”  In the linear region, vo = A (vp – vn) = A vid where A is the open-loop gain  Typically, Vcc  20 V and A > 104 linear range: -2 mV  vid = (vp – vn)  2 mV Thus, for an op amp to operate in the linear region, vp  vn (i.e. there is a “virtual short” between the input terminals.)

Achieving a “Virtual Short” Recall the voltage transfer characteristic of an op amp: Q: How does a circuit maintain a virtual short at the input of an op amp, to ensure operation in the linear region? A: By using negative feedback. A signal is fed back from the output to the inverting input terminal, effecting a stable circuit connection. Operation in the linear region enforces the virtual short circuit. Plotted using different scales for vo and vp–vn Plotted using similar scales for vo and vp–vn vo vo Vcc Vcc ~10 V slope = A >>1 slope = A >>1 ~10 V vp–vn vp–vn -Vcc -Vcc ~1 mV ~10 V

Negative vs. Positive Feedback Familiar examples of negative feedback: Thermostat controlling room temperature Driver controlling direction of automobile Pupil diameter adjustment to light intensity Familiar examples of positive feedback: Microphone “squawk” in sound system Mechanical bi-stability in light switches Fundamentally pushes toward stability Fundamentally pushes toward instability or bi-stability

Op Amp Operation w/o Negative Feedback (Comparator Circuits for Analog-to-Digital Signal Conversion) 1. Simple comparator with 1 Volt threshold:  V– is set to 0 Volts (logic “0”)  V+ is set to 2 Volts (logic “1”)  A = 100 V0 VIN 1 2 If VIN > 1.01 V, V0 = 2V = Logic “1” +  V0 VIN 1V If VIN < 0.99 V, V0 = 0V = Logic “0” 2. Simple inverter with 1 Volt threshold:  V– is set to 0 Volts (logic “0”)  V+ is set to 2 Volts (logic “1”)  A = 100 VIN 1 2 V0 If VIN < 0.99 V, V0 = 2V = Logic “1” +  V0 VIN 1V If VIN > 1.01 V, V0 = 0V = Logic “0”

Op Amp Circuits with Negative Feedback Q: How do we know whether an op amp is operating in the linear region? A: We don’t, a priori. Assume that the op amp is operating in the linear region and solve for vo in the op-amp circuit. If the calculated value of vo is within the range from -Vcc to +Vcc, then the assumption of linear operation might be valid. We also need stability – usually assumed for negative feedback. If the calculated value of vo is greater than Vcc, then the assumption of linear operation was invalid, and the op amp output voltage is saturated at Vcc. If the calculated value of vo is less than -Vcc, then the assumption of linear operation was invalid, and the op amp output voltage is saturated at -Vcc.

Op Amp Circuit Model (Linear Region) Ri is the equivalent resistance “seen” at the input terminals, typically very large (>1MW), so that the input current is usually very small: ip = –in  0 ip vp Ro io Ri vo + – A(vp–vn) in vn Note that significant output current (io) can flow when ip and in are negligible!

Ideal Op Amp Assumptions: ip = –in= 0 vp = vn Ri is large (105 W) A is large (104) Ro is small (<100 W) Simplified circuit symbol: power-supply terminals and dc power supplies not shown ip = –in= 0 vp = vn ip + – io + vp – in + vo – + vn – Note: The resistances used in an op-amp circuit must be much larger than Ro and much smaller than Ri in order for the ideal op amp equations to be accurate.

Unity-Gain Voltage Follower Circuit +  VIN vn V0(V) VIN(V) 1 2 IIN vp vp = vn  V0 = VIN ( valid as long as V –  V0  V + ) Note that the analysis of this simple (but important) circuit required only one of the ideal op-amp rules. Q: Why is this circuit important (i.e. what is it good for)? A: A “weak” source can drive a “heavy” load; in other words, the source VIN only needs to supply a little power (since IIN = 0), whereas the output can drive a power-hungry load (with the op-amp providing the power).